Late proterozoic rifting, glacial sedimentation, and sedimentary cycles in the light of windermere deposition, Western Canada

Abstract

The upper Proterozoic Windermere Supergroup of western Canada (800-570 Ma) contains the depositional record of a wedening rift system along the evolving passive continental margin of North America. Concomitant fault tectonics affected the cratonic (older than 1750 Ma) crust and superimposed middle Proterozoic pericratonic troughs (dating from about 1600 to 800 Ma). The depositional history of the Windermere Supergroup in Canada (and probably also in the adjacent United States) can be understood in terms of three major shoaling-upwards cycles which are best exposed in the Mackenzie Mountains. The first cycle (Rapitan cycle) contains abundant evidence of faulting contemporaneous with glacial sedimentation. A predominant “proglacial” siltstone facies is overlain by an “ice-marginal” diamictite complex which represents the glaciomarine grounding-line environment of a fluctuating land-based ice sheet. The second cycle (Hay Creek cycle) commences with transgressive black shale or limestone-laminite which grade upsection into a varied and cyclic shallow-water shelf assemblage of clastics and carbonates; the top of this cycle is composed of abundant carbonate olistostromes and oligomictic diamictites which are capped by a thin but regionally persistent shallow-water dolostone. The third cycle (Sheepbed-Backbone Ranges cycle) again commences with black silty shale and is topped by a shallow-water marine to nonmarine dolostone— sandstone succession of sub-Cambrian age. Possible equivalence of these three cycles with similar cycles in the Adelaide Basin of Australia and in the Sinian successions of China suggests that the three basins could have developed along the margins of a complex rift system that led to the opening of an early Paleozoic Pacific Ocean (?) basin.